EP0334796B1 - Process for the production of unsaturated halogenated hydrocarbons - Google Patents

Description

The invention relates to a process for the production of unsaturated halogenated hydrocarbons by electrolysis in the presence of certain onium compounds and metal salts.

Unsaturated halogenated hydrocarbons such as tetrafluoroethylene, chlorotrifluoroethylene, vinylidene fluoride or hexafluoropropene are of great technical importance especially for the production of fluoroplastics and inert liquids.

Halogenated olefins, especially fluorine-containing olefins, are among others. prepared by decarboxylation of fluorocarboxylic acids, pyrolysis of chlorofluorocarbons or thermal or base-catalyzed dehydrohalogenation of hydrogen-containing haloalkanes.

The dehalogenation of bromine- or chlorine-containing fluorocarbons, which is carried out, for example, with zinc in methanol, provides particularly pure products.

durchgeführt (DE-PS 28 18 066). Als Kathode dient eine Zink- bzw. eine Aluminiumplatte, jedoch sind auch andere Metallkathoden geeignet. Der Katholyt besteht aus CF₂Cl-CFCl₂, Wasser, Zinkchlorid und einem anionischen Detergens.Since this process is associated with the accumulation of large amounts of zinc salts, methods have been developed to produce the zinc electrochemically and to regenerate it after the reaction. Such a procedure is based on the reaction equation $$\text{CF₂Cl-CFCl₂ → CF₂ = CFCl + Cl₂}$$

carried out (DE-PS 28 18 066). A zinc or aluminum plate serves as the cathode, but other metal cathodes are also suitable. The catholyte consists of CF₂Cl-CFCl₂, water, zinc chloride and an anionic detergent.

In a cell divided by a perfluorinated cation exchange membrane, electrolysis is carried out at 10 to 15 V and a current density of 55 mA / cm². Chlorine is developed on the anode, which consists of platinum. The yield of CF₂ = CFCl is approximately 90% with a current yield of 81.9%.

However, the process is uneconomical since the current density is too low for a technical process, the voltage and thus the energy consumption are too high and the platinum anodes are too expensive.

Also known is the dehalogenation of organic halogen compounds such as CF₂Cl-CFCl₂ by electrochemically deposited metals with dehalogenating properties such as Zn, Sb, As, Cd and Fe (published US application 762,873). The electrolysis is carried out on electrodes made of copper in an aqueous ethanolic solution of the metal salt. The electrolytic cell itself is not divided, but anode and cathode gases are collected separately. The disadvantage of this process is above all the use of copper as an anode material, since copper is known to be not a stable material for anodes on which chloride ions are oxidized to chlorine, as is inevitably the case in an undivided electrolysis cell. In addition, the proposed electrolytic cell is not suitable for a technical process and the current density is too low to ensure that the method is carried out economically.

Another method works with even lower current densities (SU-PS 520.342). CF₂Cl-CFCl₂ is electrolyzed in aqueous emulsion in the presence of an emulsifier on graphite electrodes with a current density of 30 mA / cm². A pure product is obtained in a yield of 80%.

The disadvantage of this process is the use of the expensive and explosive conductive salt LiClO₄. In addition, damage to the cation exchange membrane occurs when the electrolysis is carried out in a technical flow cell, as described in Comparative Example 1.

The problem of membrane damage can be overcome by using porous ceramic diaphragms. In this case, however, a mixing of the anolyte and the catholyte as well as the anolyte and the catholyte exhaust gas is to be expected, so that on the one hand the olefin reacts with the anodically produced chlorine and on the other hand chlorine and the explosive chlorine oxyhydrogen gas formed on the cathode can form.

Ceramic diaphragms are used in various processes (SU-PS 230.131, Zh. Prikl. Chim. 1978, Vol. 51, pp. 701, 703). The electrolysis of chlorofluorocarbons such as CF₂Cl-CFCl₂ or CF₂Cl-CF₂Cl to chlorotrifluoroethylene and tetrafluoroethylene can be carried out in basic or neutral mixtures of water and polar organic solvents such as isopropanol, acetone or dioxane. At current densities of 17 to 34 mA / cm² and voltages from 4 to 9 V, the current yields are between 53.3 and 21.7%.

In addition to the base KOH, which is added to neutralize the resulting hydrochloric acid and thus cause an undesirable salt accumulation, the catholyte, which consists of 75 ml KOH solution in water and 75 ml isopropanol, has to be fed about 0.15 g lead nitrate per hour to maintain the activity of the lead cathode.

In addition to the problems associated with the use of ceramic diaphragms, corrosion or passivation of the cathode also occur in the processes described. The addition of bases and lead nitrate solutions to the catholyte produces not only saline but also heavy metal wastewater.

The use of a ceramic diaphragm is also cited in another literature reference (IT-OS 852.487). In this process, the metal mercury which is unsuitable for a technical process for toxicological and process engineering reasons is preferred as the cathode. In a catholyte consisting of water and water-soluble solvents such as dioxane or acetone and a buffer salt such as potassium acetate, electrolysis is carried out under potential control.

The economics of the process are greatly affected by the high salt accumulation in the wastewater, the use of mercury and the complex control of the potential of the cathode.

Also known is the electrolysis of halogenated hydrocarbons to halogen olefins on porous, hydrophobic plastic-metal composite electrodes made of, for example, copper or zinc, which is carried out in neutral or slightly basic aqueous medium or in an electrolyte made of 2-molar lithium perchlorate in water (SU-PS 702 702, Elektrochimya, 1986, vol. 22, p. 1132, CA: 105: 180351; Zh. Prikl. Chim. 1986, vol. 59, p. 1179, CA: 105: 31815). In addition to the inevitable salt accumulation and possibly the use of lithium perchlorate, the instability of the Zn electrodes is also disadvantageous since they become hydrophilic and spongy during the electrolysis.

It was therefore the task of providing an economical process for the dehalogenation of chlorofluorocarbons with the formation of chlorofluorofluorefins, which does not have the disadvantages of the above-mentioned processes, such as passivation or corrosion of the cathodes, use of toxic mercury, instability of Ion exchange membranes, risk of chlorine oxyhydrogen gas development, low current density, low current efficiency and high salt accumulation in the wastewater.

This object is achieved by the inventive method according to claim 1; preferred embodiments of this method are the subject of dependent claims 2-17.

in denen

X

Phosphor oder Stickstoff,

R⁷

Wasserstoff, Alkyl, Cycloalkyl, Aralkyl mit 1 bis 18 C-Atomen im Alkylrest und Aryl mit 6 bis 12 C-Atomen bedeutet,

R⁸

gleich R⁷ ist oder -(R⁷-O)_pR⁷ bedeutet,

R⁹

gleich R⁷, R⁸ ist oder -CH₂(Y)_qCH₂- bedeutet,

R¹⁰

-(CH₂)_p-, -CH₂-[O-(CH₂)_p]_q-O-(CH₂)₂- ist,

p

eine ganze Zahl von 1 bis 12 ist,

q

Null oder eine ganze Zahl von 1 bis 6 ist,

Y

Stickstoff, Sauerstoff, Schwefel oder -CH₂- und

Z

-OH oder ein Anion einer anorganischen oder organischen Säure bedeutet. Diese Säuren sind beispielsweise die verschiedenen Halogenwasserstoffsäuren, Schwefelsäure, Salpetersäure, salpetrige Säure, Phosphorsäure, H₃BO₃, HBF₄, HPF₆, Ameisensäure, Essigsäure und Oxalsäure.

The onium compounds B) with at least one knitted fabric or phosphorus atom are compounds of the formulas III to VI below.

in which

X

Phosphorus or nitrogen,

R⁷

Is hydrogen, alkyl, cycloalkyl, aralkyl having 1 to 18 carbon atoms in the alkyl radical and aryl having 6 to 12 carbon atoms,

R⁸

is R⁷ or - (R⁷-O) _p means R⁷,

R⁹

is R⁷, R⁸ or is -CH₂ (Y) _q CH₂-,

R¹⁰

- (CH₂) _p -, -CH₂- [O- (CH₂) _p ] _q -O- (CH₂) ₂-,

p

is an integer from 1 to 12,

q

Is zero or an integer from 1 to 6,

Y

Nitrogen, oxygen, sulfur or -CH₂- and

Z

-OH or an anion of an inorganic or organic acid. These acids are, for example, the various hydrohalic acids, sulfuric acid, nitric acid, nitrous acid, phosphoric acid, H₃BO₃, HBF₄, HPF₆, formic acid, acetic acid and oxalic acid.

Starting compounds of formula (II) are polyhalogenated alkyl compounds, preferably the dichlorides, dibromides or bromochloride addition products of corresponding olefins, which are e.g. derived from the following olefins: 1,1,2,2-tetrafluoroethylene, 1,1,2-trifluoro-2-chloroethylene, 1,1,2-trifluoroethylene, the various dichlorodifluoro, difluoro, difluorochloroethylene, 1.1 , 2-trichloro-2-fluoroethylene, fluoroethylene, the various dichlorofluoro- and chlorofluoroethylene as well as hexafluoropropene.

The compounds of formula (II) are in concentrations of 1% to 60 wt .-%, preferably 5 to 50 wt .-%, based on the total amount of the electrolyte D) in the undivided cell or the catholyte D₁) in the divided Cell used.

The method according to the invention is carried out in divided or undivided cells. Accordingly, there is a catholyte D₁) or an anolyte D₂) in divided cells, while only one electrolyte D) is present in undivided cells. These circumstances must be taken into account when interpreting the description and claims. To exchange the cells in the anode and cathode compartments, ion exchange membranes, in particular cation exchange membranes made of a polymer such as polystyrene, preferably of perfluorinated polymers with carboxyl and / or sulfonic acid groups, are used. The use of stable anion exchange membranes is also possible.

The electrolysis can be carried out in all conventional electrolysis cells, for example in beaker or plate and frame cells or cells with fixed bed or fluidized bed electrodes. Both the monopolar and the bipolar circuit of the electrodes can be used.

It is possible to carry out the electrolysis both continuously and batchwise.

Electrolysis is generally carried out on carbon cathodes. All known carbon electrode materials can therefore be used as carbon cathodes, for example electrode graphites, impregnated graphite materials, porous graphites, carbon felts, vitreous carbon and also carbon-plastic composite materials. The composite materials used are, for example, polytetrafluoroethylene and polyvinylidene fluoride. All known materials on which the corresponding anode reactions take place can be used as anode material. For example, lead, lead dioxide on lead or other carriers, platinum, titanium dioxide doped with noble metal oxides (such as ruthenium dioxide) on titanium are suitable for the development of oxygen from dilute sulfuric acid. Carbon or titanium dioxide doped with precious metal oxides on titanium are suitable, for example, for the development of chlorine from aqueous alkali metal chloride or aqueous or alcoholic hydrogen chloride solutions.

When working in divided electrolytic cells, the use of an anolyte D₂) is required. Suitable anolyte liquids are aqueous mineral acids or solutions of their salts, for example dilute sulfuric acid, hydrochloric acid, sodium sulfate or sodium chloride solutions or solutions of hydrogen chloride in alcohol.

In response to the anode, e.g. the development of halogen from aqueous or alcoholic alkali halide or hydrogen halide solutions instead.

The electrolyte D) in the undivided or the catholyte D₁) in the divided cell contains the compound of formula (II) used and consists of water, one or more organic solvents or a mixture of both. Examples of suitable organic solvents are short-chain aliphatic alcohols such as the various butanols; Diols such as propanediol, but also polyethylene glycols and their ethers; Ethers such as tetrahydrofuran, amides such as hexamethylphosphoric triamide, nitriles such as propionitrile; Ketones such as acetone; as well as sulfolane or dimethyl sulfoxide, but preferably methanol, ethanol, the various propanols, ethylene glycol, dioxane, N, N-dimethylformamide and N-methyl-2-pyrrolidone.

Under the conditions described, reactions can sometimes occur between the organic solvent and the halogen formed, which lead to the formation of undesirable by-products, such as the formation of dichlorodimethyl ether from methanol, chlorine and hydrogen chloride. These side reactions can be prevented by in a electrolyte D) or catholyte D₁), a mixture of water and at least one organic acid and / or at least one salt of this acid and that, with the exception of the organic Solvent, which contains the other additives described, is electrolyzed.

The electrolyte D) in the undivided cell or the catholyte D₁) in the divided cell are C) soluble salts of metals with a hydrogen overvoltage of at least 0.25 V, based on a current density of 100 mA / cm²) in concentrations of 10⁻ ⁵ to 5 wt .-%, preferably 10⁻³ to 5 wt .-%, each based on the total amount of the electrolyte or catholyte, added.

NO₃⁻, CH₃COO⁻ und PO $\genfrac{}{}{0ex}{}{\text{3-}}{\text{4 }}$

. Die Salze können direkt zugesetzt oder auch z.B. durch Zugabe von löslichen Oxiden oder Carbonaten in der Lösung erzeugt werden. Bei der Wahl der Anionen ist darauf zu achten, daß mit den Kationen der vorgenannten Metalle keine im Elektrolyten unlöslichen Verbindungen gebildet werden.The preferred anions of these salts are Cl⁻, SO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4th}}$

NO₃⁻, CH₃COO⁻ and PO $\genfrac{}{}{0ex}{}{\text{3-}}{\text{4th}}$

. The salts can be added directly or, for example, can be generated in the solution by adding soluble oxides or carbonates. When choosing the anions, care must be taken to ensure that no compounds which are insoluble in the electrolyte are formed with the cations of the aforementioned metals.

In addition to C), the metal salts, one or more compounds B) with at least one nitrogen or phosphorus atom according to the formulas (III) to (VI) in concentrations of 10oly to 10% by weight are added to the electrolyte or the catholyte. , preferably 10⁻⁴ to 5 wt .-%, based on the total amount D) or D₁) added.

Particularly suitable compounds of the formulas (III) to (VI) are tetramethyl, tetraethyl, tetrapropyl-, tetrabutylammonium or -phosphonium-, benzyl-, octyl, decyl, dodecyl, tetradecyl-, hexadecyl, octadecyltrimethylammonium or -trimethylphosphonium, dio , Didecyl-, Didodecyl, Ditetradecyl, Dihexadecyl, Dioctadecyldimethylammonium or -dimethylphosphonium, Methyltrioctylammonium and mixtures thereof. However, primary, secondary and tertiary amines can also be used, from which the onium compounds are formed in the course of the electrolysis. The nature of the anions of the compounds B) is irrelevant for the process; preference is given to the halide, sulfate, tetrafluoroborate and hydroxide ions.

mit r= 0-2;
C₁-C₅-Alkansulfonsäure wie Methan- und Äthansulfonsäure sowie halogenierte Säuren z.B. Trifluormethansulfonsäure; aromatische Sulfonsäuren wie Benzolsulfonsäure oder Toluolsulfonsäure sowie C₁-C₅-Alkanphosphonsäure wie Methan- oder Äthanphosphonsäure geeignet.To increase the conductivity, the catholyte in the divided cell or the electrolyte in the undivided cell as component E) can have at least one inorganic and / or organic acid and salts thereof in concentrations of 5 to 60, preferably 10 to 50,% by weight to be added to the total amount of the electrolyte. Hydrochloric, boric, phosphoric, sulfuric or tetrafluoroboric acid can be used as the inorganic acids, but preference is given to the organic acids. As organic acids are water-soluble mono- or dicarboxylic acids, for example C₁-C₅ alkane carboxylic acids such as formic, acetic, propionic, butter or valeric acid and halogenated acids such as chloroacetic acid or trifluoroacetic acid; Malonic or succinic acid, ether carboxylic acids such as methoxy or ethoxy acetic acid and fluorinated ether carboxylic acids of the formula

with r = 0-2;
C₁-C₅ alkanesulfonic acid such as methane and ethanesulfonic acid and halogenated acids, for example trifluoromethanesulfonic acid; aromatic sulfonic acids such as benzenesulfonic acid or toluenesulfonic acid and C₁-C₅-alkanephosphonic acid such as methane or ethanephosphonic acid are suitable.

The salts of the acids mentioned are the ammonium, sodium, potassium and / or C₁-C₄-tetraalkylammonium salts.

mit r= 0 und 1
sowie die Methan- und Äthansulfon- und -phosphonsäure und deren Salze.Formic, vinegar, chloroacetic, trifluoroacetic, propionic, methoxyacetic and fluorinated ether carboxylic acids are preferred

with r = 0 and 1
as well as the methane and ethanesulfonic and phosphonic acid and their salts.

In the case of electrolysis in undivided cells, compounds can be added to the electrolyte which are oxidized at a more negative potential than the released halogen ions in order to avoid the formation of the free halogen. For example, compounds of the formulas (III) and (IV) are suitable in which the anion Z is radicals of oxalic acid, methoxyacetic acid, glyoxylic acid, formic acid and / or hydrochloric acid, e.g. the tetramethyl and tetraethylammonium compounds of the acids mentioned.

In general, the electrolysis is carried out at atmospheric pressure. Since some of the suitable organic acids or their salts are not sufficiently soluble under the conditions described, in particular at low temperatures, and some of the starting products have very low boiling points, it may be necessary to carry out the electrolysis under increased pressure of up to 10 bar, preferably Carry out up to 7 and in particular up to 5 bar and, if appropriate, elevated temperature. The current density in the electrolysis is generally 1 to 600 mA / cm², preferably 10 to 500 mA / cm², in particular 20 to 400 mA / cm².

The electrolysis temperature is in the range from -40 ° C. to the boiling point of the electrolyte or catholyte used, preferably from -30 ° C. to 90 ° C., in particular from -10 ° to 80 ° C.

Electrolysis under increased pressure makes it possible to shift the boiling point of the electrolyte or catholyte to higher values, in order to improve the solubility of the starting compounds and of the acids or salts.

The pH of the electrolyte can be varied between 0 and 14 over the pH range mentioned.

However, electrolysis at a pH of less than 7 is advantageous, since under these conditions the metal ions used do not form poorly soluble compounds which can destroy the cation exchange membrane of a divided cell. The electrolysis is carried out in particular at a pH between 5 and 0.2.

The reaction products generally leave the electrolysis arrangement in gaseous form or under elevated pressure or also in condensed form and are in suitable vessels, e.g. Cold traps, caught.

The electrolyte or catholyte is worked up, the isolation of non-gaseous products and the recovery of unreacted halogen fluorocarbons is carried out by extraction and / or distillation in a known manner. The added metal salts and the compounds of the formulas (III) to (VI) or the acids or salts contained in electrolytes or catholytes can include be fed back to the electrolysis because the starting products and the hydrohalic acids formed in most cases boil lower than the organic acids and can thus be separated off easily. The hydrohalic acids can be added to the anolyte where they are oxidized to halogen.

The products obtained by the process according to the invention are suitable as a starting material for the production of fluorine-containing polymers.

Two types of electrolytic cells were used for the following examples, but divided cells were always used.

Electrolysis cell 1:

So-called circulation cell with 0.02 m² electrode area. Electrode graphite or impregnated graphite (®Diabon N from Sigri, Meitingen, Germany) was used as the cathode and graphite or a platinum plate impregnated as the anode. Anolyte: aqueous 15 to 35% hydrochloric acid, saturated methanolic hydrochloric acid or 0.5 to 2N aqueous sulfuric acid. The electrode spacing was 4 mm and nets made of polyethylene were used as spacers. The cation exchange membrane was a two- or single-layer membrane made from a copolymer of a perfluorosulfonylethoxy vinyl ether and tetrafluoroethylene (type ®Nafion 324 or 423 from DuPont, Wilmington, Dela., USA).

Electrolytic cell 2:

Uncovered glass pot cell with a volume of 350 ml;
Cathode (®Diabon N from Sigri, Meitingen, Germany);
Anode: platinum mesh, graphite or lead plate (20 cm²);
Cathode area: 12 cm²; Electrode distance: 1.5 cm;
Anolyte: as in electrolytic cell 1;
Cation exchange membrane: Nafion 324;
Mass transfer: by magnetic stirrer.

Examples

• 1) Electrolytic cell: 1
  Starting catholyte: 3 l methanol, 200 ml dilute hydrochloric acid, 5 g [CH₃ (C₈H₁₇) ₃N] ^⊕ Cl ^ϑ , 2 g Pb (OCOCH₃) ₂ · 2H₂O and 710 g CF₂Cl-CFCl₂.
  In the course of the continuously operated electrolysis, 2.2 kg of CF₂Cl-CFCl₂ and 4 g of lead acetate were added to the catholyte in portions. The resulting CF₂ = CFCl left the electroysis room in the gaseous state and was condensed in cold traps at -78 ° C.
  Temperature: 38 to 25 ° C, voltage: 9.5 to 7 V,
  Current density: 250 mA / cm², flow: 1400 l / h.
  After a charge consumption of 331 Ah, the electrolysis was interrupted and the catholyte was worked up by distillation. 1150 g of CF₂Cl-CFCl₂ were recovered. The condensed gas was subjected to cold distillation. Yield 636 g CF₂ = CFCl (97.6%). The current yield was 88.4%.
• 2) Electrolysis cell: 1
  Starting catholyte: 2 l of ethanol, 200 ml of concentrated hydrochloric acid, 2 g of Pb (OCOCH₃) ₂ · 2H₂O, 2 g of [(C₄H₉) ₄P] ^⊕ Br ^ϑ , 600 g of CF₂Cl-CFCl₂.
  Temperature: 28 to 23 ° C, voltage: 12 to 9.5 V,
  Current density: 200 mA / cm², flow 1000 l / h.
  After a charge consumption of 132 Ah, 179 g of CF₂ = CFCl (99%) were obtained after purification by distillation.
  The current yield was 67%.
• 3) Electrolytic cell 2, starting catholyte: 150 ml dimethylformamide, 10 ml concentrated hydrochloric acid, 0.5 g Bi (NO₃) ₃, 0.5 g [CH₃ (C₈H₁₇) ₃N] ^⊕ Cl ^ϑ , 70 g CF₂Br-CFClBr
  Temperature: 40 to 28 ° C, voltage: initially 47 V, then decreasing to 16 V, current density: 250 mA / cm².
  After a charge consumption of 9 Ah, 17.1 g of CF₂ = CFCl (99.3%) were obtained after purification by distillation. 29.6 g of CF₂Br-CFBrCl were recovered. The current yield was 85.3%.
• 4) Electrolysis cell 2, with the change that the anode and cathode compartments are separated from one another by an anion exchanger membrane of the type ®Neosepta AV-4T, from Tokuyama-Soda, Tokuyama-City, Japan.
  Starting catholyte: 200 ml of methanol, 20 ml of concentrated hydrochloric acid, 1 g. Cr₂ (SO₄) ₃, 2 g [(C₁₆H₃₃) ₂N (CH₃) ₂] ^⊕ Cl ^ϑ , 70 g CF₂Br-CFClBr.
  Temperature: 20 to 22 ° C, voltage: 10 to 11 V,
  Current density: 167 mA / cm².
  After a charge consumption of 8 Ah with a conversion of 35.6 g CF₂Br-CFBrCl after the distillation 15 g (99.3%) CF₂ = CFCl were obtained.
• 5) Electrolysis cell 2, with the change that a cation exchange membrane made of non-fluorinated polymers of the type ®Selemion LMV / CHR from Asahi Glass, Tokyo, Japan was used.
  Starting catholyte: 150 ml methanol, 10 ml HBF₄ (50% in water), 0.5 g AgNO₃, 1 g [(C₁ (H₃₃) N (CH₃) ₃] ^⊕ Br ^ϑ , 70 g CF₂Br-CFClBr.
  Temperature: 20 to 25 ° C, voltage: 10 to 12 V,
  Current density: 167 mA / cm².
  After a charge consumption of 8 Ah and a conversion of 35.5 g CF₂Br-CFClBr 14.1 g (92.8%) CF₂ = CFCl were obtained after the distillation.
• 6) Electrolytic cell 2:
  Starting catholyte: 150 ml dioxane, 50 ml N, N-dimethylformamide, 15 ml HBF₄ (50% in water), 1 g [(C₄H₉) ₄N] ^⊕ HSO₄ ^ϑ , 0.5 g TlCl, 70 g CF₂Br-CFBrCl.
  Temperature: 35 to 30 ° C, voltage: 40 to 20 V,
  Current density: 250 mA / cm².
  After a charge consumption of 11.5 Ah and a conversion of 35 g CF₂Br-CFClBr 14 g (94.2%) CF₂ = CFCl were obtained after purification by distillation.
• 7) Electrolysis cell 1, with the change that the anode consisted of glass-like carbon (®Sigradur K, from Sigri, Meitingen, Germany). A 25% solution of HBF₄ in water served as the anolyte liquid.
  Starting catholyte: 3 l methanol, 200 ml dil. Hydrochloric acid, 2 g Pb (OCOCH₃) ₂ · 2H₂O, 3 g [CH₃ (C₈H₁₇) ₃N] ^⊕ Cl ^ϑ , 1000 g CF₂Br-CHFCl.
  During the electrolysis, 835 g of CF₂Br-CHFCl were added to the catholyte. The electrolysis product was continuously collected in cold traps at -78 ° C.
  Temperature: 36 to 49 ° C, voltage: 9.5 to 8 V,
  Current density: 250 mA / cm², flow: 1500 l / h
  After a charge consumption of 476 Ah, the electrolysis was interrupted and the catholyte was worked up by distillation. 194 g of CF₂Br-CFHCl were recovered. 680 g of CF₂ = CFH were obtained (yield: 99%). The current yield was 93.9%.
• 8) Electrolysis cell 1:
  Starting catholyte: 2.5 l ethanol, 200 ml concentrated hydrochloric acid, 500 g CF₂Cl-CCl₃, 1 g Pb (OCOCH₃) ₂ · 2H₂O, 2 g [(C₄H₉) ₄N] ^⊕ Br ^ϑ .
  Temperature: 28 to 46 ° C, voltage: 13 to 7.5 V,
  Current density: 200 mA / cm², flow: 500 l / h
  During the electrolysis, the product was continuously distilled out of the catholyte at a negative pressure of 400 mbar.
  After a charge consumption of 300 Ah, 263 g (86.9%) CF₂ = CCl₂ were obtained and 35 g CF₂Cl-CCl₃ were recovered from the catholyte.
• 9) Electrolysis cell 2:
  Starting electrolyte: 180 ml methanol, 5 g [(CH₃) ₄P] ^⊕ BF₄ ^ϑ ,
  20 g CF₃-CFBr-CF₂Br, 0.7 g CuSO₄.
  Temperature: 30 to 48 ° C, voltage: 28 to 10 V,
  Current density: 166 mA / cm².
  After a charge consumption of 2.87 Ah, 5.8 g (72.5%) CF₃ = CF-CF₂ were obtained.
• 10) Electrolytic cell 2:
  28.7 g CCl₃-CFCl-CFCl₂, 200 ml methanol, 0.5 g
  Pb (OCOCH₃) ₂ · 2H₂O, 4 g [(CH₃) ₄N] ^⊕ Cl ^ϑ .
  Current density: 170 mA / cm², voltage: 60 V at the beginning, then falling to 9 V, temperature: 30 ° C.
  After a charge consumption of 22.2 Ah, the catholyte was mixed with 500 ml of water and extracted with pentane. After distillation of the pentane, the following were obtained:
  5.97 g CCl₃CFCl-CCl₃
  15.41 g (89.9%) CCl₂ = CF-CFCl₂
  0.09 g ClFC = CF-CCl₃
  0.07 g HFC = CF-CCl₃
  0.09 g CHCl₂-CFCl-CFCl₂
• 11) Electrolytic cell 2:
  Starting catholyte: 20 g CF₂Br-CFBr-CH₂-CH₃, 100 ml methanol,
  2 g [(CH₃) ₄N] ^⊕ Cl ^ϑ , 0.5 g [(C₄H₉) ₄N] ^⊕ HSO₄ ^ϑ , 0.2 g Pb (OCOCH₃) ₂ · 2H₂O.
  Charge consumption: 4.93 Ah, temperature: 40 to 24 ° C,
  Current density: 164 to 41 mA / cm², voltage: 13 to 4.5 V
  Refurbishment as in example 10.
  Electrolysis result:
  0.8 g CF₂Cl-CFBr-CH₂-CH₃
  8.3 g (79.6%) CF₂ = CF-CH₂-CH₃
• 12) Electrolytic cell 2:
  Starting catholyte: 10 g CF₂Br-CFCl-CH₂-CH₂-Br, 100 ml methanol, 2 g [(CH₃) ₄N] ^⊕ Cl ^ϑ , 0.5 g [C₄H₉] ₄N] ^⊕ Br ^ϑ , 0.2 g Pb (OCOCH₃ ) ₂ · 2H₂O.
  Charge consumption: 2.1 Ah, temperature: 12 to 10 ° C,
  Current density: 41 mA / cm², voltage: 9 to 7.2 V.
  Refurbishment as in example 10.
  Electrolysis result:
  0.1 g CF₂Br-CFCl-CH₂-CH₂Br
  2.73 g (67.7%) CF₂ = CF-CH₂-CH₂-Br
• 13) Electrolytic cell 2:
  Starting catholyte: 10 g CF₂Br-CFCl- (CH₂) ₃-CH₂-Br, 100 ml methanol, 2 g [(CH₃) ₄N] ^⊕ Cl ^ϑ , 0.5 g [C₄H₉] ₄N] ^⊕ HSO₄ ^ϑ , 0.2 g Pb (OCOCH₃) ₂ · 2H₂O.
  Charge consumption: 1.66 Ah, temperature: 17 to 15 ° C,
  Current density: 82 to 41 mA / cm², voltage: 13 to 7 V.
  Refurbishment as in example 10.
  Electrolysis result:
  0.49 g CF₂Br-CFCl- (CH₂) ₃-CH₂Br
  4.9 g (75%) CF₂ = CF- (CH₂) ₃-CH₂Br
• 14) Electrolytic cell 2:
  Starting electrolyte: 200 ml isopropanol, 100 ml N, N-dimethylformamide, 80 ml water, 8 g [(CH₃) ₄N] ^⊕ OSO₂-OCH₃ ^ϑ , 1.6 g Pb (OCOCH₃) ₂ · 2H₂O, 20 g CF₂Cl-CFCl- CF₂Cl.
  Temperature: 46 to 35 ° C, current density: 166 to 83 mA / cm²,
  Voltage: 20 to 6 V, charge consumption: 4.32 Ah.
  During the electrolysis, 6.9 g of condensate were collected in a cold trap. Product still dissolved in the catholyte was obtained under reduced pressure (40 ° C., 400 mbar). A total of 7.8 g (87.1%) CF₂Cl-CF = CF₂ with a boiling point of 7.5 ° C was obtained.
• 15) Electrolytic cell 1:
  Starting catholyte: 1000 cc acetic acid (100%), 500 g sodium acetate, 3000 ccm water, 1000 g CF₂ClCFCl₂, 2 g Pb (OCOCH₃) ₂ · 2H₂O, 2 g [CH₃ (C₈H₁₅) ₃N] ^⊕ Cl ^ϑ .
  Temperature: 35-45 ° C, current density: 200 mA / cm², voltage: 10-9 V,
  Flow: 400 l / h, pH: 4.2,
  The resulting CF₂ = CFCl left the cathode space in gaseous form and was condensed in cold traps at -78 ° C.
  After a charge consumption of 195 Ah and a hydrogen evolution of 6.6 liters, the electrolysis was stopped and 351 g of unreacted starting product were recovered from the catholyte by distillation.
  The condensed gas was subjected to cold distillation, whereby 372 g (95.1%) CF₂ = CFCl was obtained.
  The current yield was 87.8%.
• 16) Electrolysis cell 1:
  Starting catholyte: 2000 cc acetic acid (100%), 2000 ccm water, 500 ccm HCl conc., 1550 g CF₂Cl-CF₂Cl / CF₃-CFCl₂ (mixing ratio 82/18), 2 g Pb (OCOCH₃) ₂ · 2 H₂O 2 g [CH₃ (C₈H₁₅) ₃N] ^⊕ Cl ^ϑ .
  Temperature: 0-5 ° C, current density: 200 mA / cm², voltage: 11-9 V, flow: 400 l / h.
  The product left the catholyte in the gaseous state. The entrained starting product and by-product formed were condensed in two series-connected cold traps at -78 ° C. The product was caught in a cold trap at -196 ° C.
  After a charge consumption of 262 Ah and a hydrogen evolution of 40.3 liters, the electrolysis was stopped and the catholyte was worked up by distillation. The contents of the cold traps were subjected to cold distillation. A total of 192.7 g of CF₂ = CF₂ (yield: 80.2%, based on the converted CF₂Cl-CF₂Cl), 956 g of starting product and 100 g of CF₃-CHClF were obtained.
• 17) Electrolytic cell 1
  Starting catholyte: 500 g
  
  1000 g water, 600 g CF₂ClCFCl₂, 2 g Pb (OCOCH₃) ₂ · 2H₂O, 5 g [(C₄H₉) ₄ P] ^⊕ Cl ^⊖ , 1 g [CH₃ (C₈H₁₅) ₃N] ^⊕ Cl ^ϑ
  Temperature: 25 - 45 ° C, current density: 200 mA / cm², voltage: 14 - 12 V, flow: 400 l / h, pH: 1.4-0.6
  The resulting CF₂ = CFCl left the cathode space in gaseous form and was condensed in cold traps at -78 ° C.
  After a charge consumption of 102 Ah and a hydrogen evolution of 82.1 liters, the electrolysis was stopped and 420 g of unreacted starting product were recovered from the catholyte by distillation.
  The amount of the condensed gas was 97 g (yield 87.8%). The current efficiency was 43.2%.
• 18) Electrolysis cell 1
  Starting catholyte: 350 g methanesulfonic acid (70% in water), 300 g 20% KOH solution, 1000 ml water, 300 g CF₂ClCFCl₂, 2 g Pb (OCOCH₃) ₂ · H₂O, 1 g [(C₄H₉) ₄P] ^⊕ Cl ^⊖ temperature : 30-45 ° C, voltage: 12-9 V, current density: 150 mA / cm², flow: 400 l / h, pH: 0.8-0.2.
  The resulting CF₂ = CFCl left the cathode space in gaseous form and was condensed in cold traps at -78 ° C. In the course the electrolysis were replenished to the catholyte 110 g / h, ie a total of 600 g CF₂Cl-CFCl₂. After a charge consumption of 162 Ah and hydrogen evolution of 5.1 liters, the electrolysis was stopped and 674 g of unreacted starting product were recovered from the catholyte by distillation.
  The condensed gas was subjected to cold distillation. Yield 285 g CF₂ = CFCl (81.8%). The current yield was 76.14%.
• 19) Electrolysis cell 1:
  Starting catholyte: 1000 g chloroacetic acid, 500 g sodium chloroacetate, 2000 ml water, 200 g CF₂ClCFCl₂, 2 g Pb (OCOCH₃) ₂ · 2H₂O, 5 g [(C₄H₉) ₄N] ^⊕ Cl ^⊖ temperature: 25-45 ° C, voltage: 8 -6.3 V, current density: 200 mA / cm², flow rate: 3100 l / h, pH value: 4.45-0.5
  The resulting CF₂ = CFCl left the cathode space in gaseous form and was condensed in cold traps at -78 ° C.
  During the electrolysis 200 g / h, ie a total of 2000 g CF₂Cl-CFCl₂ were added to the catholyte. After a charge consumption of 402 Ah and a hydrogen evolution of 19.3 liters, the electrolysis was stopped and 903 g of unreacted starting product were recovered from the catholyte by distillation.
  The condensed gas was subjected to cold distillation. Yield 668 g CF₂ = CFCl (81.7%). The current yield was 72.7%.

Comparison 1)

In the electrolytic cell 1, a starting catholyte from 2.5 kg of water, 100 g of concentrated hydrochloric acid, 808 g of CF₂Cl-CFCl₂ and 1 g of the cationic emulsifier ^(R) Dodigen 1490 (Hoechst AG, Frankfurt aM, Federal Republic of Germany) was electrolyzed.
Temperature: 30 to 34 ° C, voltage: 5.5 to 8 V,
Current density: 150 mA / cm², flow 750 to 1300 l / h.

Concentrated hydrochloric acid in water was oxidized to chlorine on an anode made of electrode graphite. Anode and cathode compartments were separated by a Nafion 324 cation exchange membrane.

At the beginning of the electrolysis, the voltage was 5.5 V. After a charge consumption of 45 Ah, the voltage had risen to 7.1 V and continued to increase by approximately 0.1 V per minute. 74% of the charge was used to develop hydrogen from protons.

After the electrolysis was stopped, it was found that the cation exchange membrane was very badly damaged.

Comparison 2)

Electrolytic cell 1
Starting catholyte: 2000 ml of methanol, 100 ml of concentrated hydrochloric acid, 500 g of CF₂Cl-CFCl₂

Temperature: 31 to 34 ° C, current density: 200 mA / cm²,
Voltage: 8 to 7.5 V, flow rate 500 l / h.

The amount of charge used to discharge protons to hydrogen increased from 28% to 65% in one hour. After adding 1 g of Pb (OCOCH₃) ₂ · 2H₂O to the catholyte, the evolution of hydrogen decreased to a proportion of 37%, which rose to 47% within 15 minutes. After adding 2 g [(C₄H₉) ₄N] ^⊕ Br⁻ only 14% of the charge was used for the development of hydrogen.

After a charge consumption of 216 Ah, 238 g (92%) CF₂ = CFCl were obtained.

It can be seen from comparison 2 that only the use of carbon cathodes using a combination of metal salts and compounds of the formulas (III) to (VI) enables electrolysis to be carried out economically.

Claims (17)

1. A process for the preparation of compounds of the formula

   

   by electrolyzing a compound A) of the formula

   

   at a carbon electrode, in which, in the formula (II), the

   R¹s   are, independently of one another, hydrogen, chlorine or fluorine, the

   R²s   are R¹ or -C(R¹)₂-R³, or the grouping [C(R¹)₂]_m-C(R¹)₂ is two of the radicals R²,

   R³   is -(CH₂)_n-CH₂-R⁵, -(CF₂)_n-CH₂-R⁵, -(CF₂)_n-CF₂-R⁵ or C₁-C₁₂-alkyl which is partly or completely fluorinated, the

   R⁴s   are, independently of one another, chlorine, bromine or iodine,

   R⁵   is R¹, bromine, iodine, -CO-R⁶ or -SO₂-R⁶,

   R⁶   is -OH, -O-alkyl having 1 to 6 carbon atoms in the alkyl radical, fluorine or chlorine,

   m and n independently are zero or an integer from 1 to 12, preferably 1 to 6, and at least one R¹ is fluorine,
   in a divided or undivided electrolysis cell in the presence of B) at least one onium compound containing at least one nitrogen or phosphorus atom, and C) at least one soluble metal salt having a hydrogen overvoltage greater than 0.25 volts, relative to a current density of 100 mA/cm², in D) an electrolyte and E) in the absence or presence of 5 to 60% by weight, relative to the total amount of the electrolyte, of at least one inorganic and/or organic acid and/or salts thereof, under atmospheric pressure or under an elevated pressure of up to 10 bar at a current density of 1 to 600 mA/cm² and at a temperature within the range from -40°C to the boiling point of the electrolyte.
2. The process as claimed in claim 1, wherein as component C) salts of the metals lead (Pb), chromium (Cr), copper (Cu), silver (Ag), thallium (Tl) and bismuth (Bi) are employed, the anions being Cl⁻, SO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4 }}$

   

   , NO₃⁻, CH₃COO⁻ and PO $\genfrac{}{}{0ex}{}{\text{3-}}{\text{4 }}$

   

   .
3. The process as claimed in claim 1 or 2, wherein the electrolysis temperature is -30 to 90°C, especially -10 to 80°C, the current density is 10 to 500 mA/cm², especially 20 to 400 mA/cm², and the pressure is up to 7 bar, especially up to 5 bar.
4. The process as claimed in one or more of claims 1 to 3, wherein the dichlorides, dibromides or bromochloride addition products of appropriate olefins are employed as the compounds of the formula (II).
5. The process as claimed in claim 4, wherein the dichlorides, dibromides or bromochloride addition products of 1,1,2,2-tetrafluoroethylene, 1,1,2-trifluoro-2-chloroethylene, 1,1,2-trifluoroethylene, the various dichlorodifluoroethylenes, difluoroethylenes or difluorochloroethylenes, 1,1,2-trichloro-2-fluoroethylene, fluoroethylene, the various dichlorofluoroethylenes and chlorofluoroethylenes and hexafluoropropene are employed.
6. The process as claimed in one or more of claims 1 to 5, wherein the onium compounds B) employed are compounds of the formulae

   

   

   

   

   in which

   X   is phosphorus or nitrogen,

   R⁷   is hydrogen, alkyl, cycloalkyl, aralkyl having 1 to 18 carbon atoms in the alkyl radical and aryl having 6 to 12 carbon atoms,

   R⁸   is the same as R⁷ or denotes -(R⁷-O)_pR⁷,

   R⁹   is the same as R⁷ or R⁸ or denotes -CH₂(Y)_qCH₂-,

   R¹⁰   is -(CH₂)_p-, -CH₂-[O-(CH₂)_p]_q-O-(CH₂)₂-,

   p   is an integer from 1 to 12,

   q   is zero or an integer from 1 to 6,

   Y   is nitrogen, oxygen, sulfur or -CH₂- and

   Z   is -OH or an anion of an inorganic or organic acid.
7. The process as claimed in one or more of claims 1 to 6, wherein electrode graphite, impregnated graphite, porous graphite, carbon felts, vitreous carbon or carbon-plastic composite materials are employed as the carbon electrode.
8. The process as claimed in one or more of claims 1 to 7, wherein the electrolysis is carried out continuously or discontinuously.
9. The process as claimed in one or more of claims 1 to 8, wherein the electrolysis is carried out in divided electrolysis cells in which a catholyte D₁) and anolyte D₂) are present.
10. The process as claimed in one or more of claims 1 to 9, wherein water and as component E) 10 to 50% by weight of at least one organic acid and/or salts thereof are employed.
11. The process as claimed in claim 10, wherein the organic acid employed is formic acid, acetic acid, chloroacetic acid, methanesulfonic acid, methanephosphonic acid or the fluorinated ether-carboxylic acids

    

    in which r = 0 and 1.
12. The process as claimed in claim 10, wherein the ammonium, sodium, potassium or tetraalkylammonium salts, having 1 to 4 carbon atoms in the alkyl radical, of the acids E) are employed.
13. The process as claimed in one or more of claims 1 to 9, wherein at least one organic solvent, water or a mixture of both is employed as the electrolyte D) or as the catholyte D₁).
14. The process as claimed in claim 13, wherein the organic solvent is methanol, ethanol, the various propanols, ethylene glycol, dioxane, N,N-dimethylformamide and N-methyl-2 -pyrrolidone.
15. The process as claimed in one or more of claims 1 to 14, wherein the compounds of the formula (II) are employed in amounts of 1% to 60%, preferably 5 to 50%, relative to the total amount of the electrolyte D) or the catholyte D₁).
16. The process as claimed in one or more of claims 1 to 15, wherein the salts C) are employed in amounts from 10⁻⁵ to 5% by weight, preferably from 10⁻³ to 5% by weight, in each case relative to the total amount of the electrolyte or catholyte.
17. The process as claimed in one or more of claims 1 to 16, wherein the compounds B) are added in amounts from 10⁻⁵ to 10% by weight, preferably from 10⁻⁴ to 5% by weight, relative to the total amount of D) or D₁).